KEY STEPS

Compendium on methods and tools to evaluate impacts of, and vulnerability and adaptation to, climate
change

Weather Generators

Description

Weather generators are not, strictly speaking, downscaling techniques, but are often used in
conjunction with the techniques outlined in this section.

A weather generator is a statistical model used to generate realistic daily sequences of weather
variables — precipitation, maximum and minimum temperature, humidity, etc. Such data are
often referred to as synthetic data. Usually precipitation sequences are generated first, and other
data sequences are derived using statistical relationships between these data and precipitation,
with different relationships used for wet and dry days.

Precipitation is divided into an occurrence process (i.e., whether the day is wet or dry) modeled
as a Markov chain, and an amount process (the amount of precipitation on a wet day) sampled
randomly from an appropriate distribution, such as a Gamma distribution. By using different random
seeds, a large number of sequences can be generated, all of which have the same statistical
properties as the original data used to calibrate the statistical model — akin to
realizations from a set of parallel universes. This is a crucial factor in assessing uncertainties
associated with the chaotic nature of daily weather variability. The SDSM software has a weather
generator component.

Appropriate Use

Weather generators are used whenever impacts models require small-scale data on a daily time scale,
provided suitable observed data are available to derive the statistical relationships.

Appropriate observed data to calibrate and validate the statistical model(s). GCM data for future
climate to drive the model(s).

Ease of Use

There are a number of weather generator software packages requiring different levels of expertise for
their use (see References below). The user-friendly software in SDSM’s weather generator
component is largely self explanatory and comes with comprehensive instructions for use.

Training Required

Requires little training for those familiar with basic climate science